This invention relates to magnetorheological fluids.
Magnetorheological (MR) fluids are substances that exhibit an ability to change their flow characteristics by several orders of magnitude and in times on the order of milliseconds under the influence of an applied magnetic field. An analogous class of fluids are the electrorheological (ER) fluids which exhibit a like ability to change their flow or Theological characteristics under the influence of an applied electric field. In both instances, these induced Theological changes are completely reversible. The utility of these materials is that suitably configured electromechanical actuators which use magnetorheological or electrorheological fluids can act as a rapidly responding active interface between computer-based sensing or controls and a desired mechanical output. With respect to automotive applications, such materials are seen as a useful working media in shock absorbers, for controllable suspension systems, vibration dampers in controllable powertrain and engine mounts and in numerous electronically controlled force/torque transfer (clutch) devices.
MR fluids are noncolloidal suspensions of finely divided (typically one to 100 micron diameter) low coercivity, magnetizable solids dispersed in a base carrier liquid such as a mineral oil, synthetic hydrocarbon, water, silicone oil, esterified fatty acid or other suitable organic liquid. MR fluids have an acceptably low viscosity in the absence of a magnetic field but display large increases in their dynamic yield stress when they are subjected to a magnetic field of, e.g., about 0.5 to greater than 1.0 Tesla. At the present state of development, MR fluids appear to offer significant advantages over ER fluids, particularly for automotive applications, because the MR fluids are less sensitive to common contaminants found in such environments, and they display greater differences in Theological properties in the presence of a modest applied field, in particular, higher yield strengths and greater damping forces.
MR fluids contain noncolloidal solid particles that are often seven to eight times more dense than the liquid phase in which they are suspended. A typical MR fluid in the absence of a magnetic field has a readily measurable viscosity that is a function of its vehicle and particle composition, particle size, the particle loading, temperature and the like. However, in the presence of an applied magnetic field, the suspended particles appear to align or cluster and the fluid drastically thickens or gels. Its effective viscosity then is very high and a larger force, termed a yield stress, is required to promote flow in the fluid.
The magnetizable solid is typically particles of iron, cobalt, nickel or magnetic alloys thereof. The presently preferred magnetizable solid for automotive applications is carbonyl iron, which is a high purity iron with soft magnetic properties. The traditional methods of producing powdered iron are the carbonyl process, inert gas atomization and water atomization.
The carbonyl process involves the thermal decomposition of iron pentacarbonyl that yields high purity iron. The particles are smooth and generally spherical, with diameters typically in the range of 1-10 xcexcm. However, carbonyl iron is liable to oxidize in use, in part due to its high level of purity. Oxidation of the carbonyl iron has been observed in MR fluids used in fan clutch and shock absorber applications, for example. Oxidation can occur as a result of exposure to high temperatures and/or moisture. Carbonyl iron powders typically begin to oxidize in air at temperatures well below 200xc2x0 C. In a clutch application, for example, the MR fluid often reaches over 200xc2x0 C. Oxidation of the iron particles can reduce the magnetorheological effect of the fluid by as much as 20% or more. Iron oxide exhibits poorer magnetic properties than pure carbonyl iron. Moreover, the yield stress for the MR fluid decreases over time, and this is believed to be a result of one or both of the oxidation of the carbonyl iron particles or a change in the shape and size distribution of the particles. This reduction in effectiveness can severely affect device performance.
Inert gas atomization produces spherical iron particles, but is relatively expensive due to the use of inert gases, such as argon, xenon, etc. Thus, the market lacks commercial suppliers of inert gas atomized iron particles. Water atomization of iron typically yields irregular, large particles. However, the process can be controlled to yield spherical, smooth particles of small diameters, and is relatively inexpensive compared to inert gas atomization and the carbonyl process. Two commercial sources for smooth, spherical, small diameter water atomized iron particles include Hoeganaes Corporation (N.J.) and Hoganas AB (Sweden). Water atomized iron powder has only recently become available, however, and thus is not currently used commercially in the MR fluid market. Carbonyl iron continues to be used, and oxidation of the magnetizable particles continues to be a problem with respect to the effectiveness of the MR fluids under long-term use.
There is thus a need to increase the resistance of MR fluids to oxidation to prevent reduction in MR fluid performance.
The present invention provides a magnetorheological fluid formulation that is resistant to oxidation and corrosion and maintains a high yield stress throughout its use under an applied magnetic field. The fluid formulation comprises a suspension of magnetizable stainless steel particles dispersed in a liquid vehicle. The stainless steel is either a ferritic grade or preferably a martensitic grade. The stainless steel powder is produced by a controlled water atomization process, which results in generally smooth, spherical particles having a mean diameter in the range of 8-25 xcexcm. Alternatively, a controlled inert gas atomization process could also be used to produce powders of the desired morphology and size distribution.